Electronic track circuits



March 29, 1960 l.. DUTEH.

ELECTRONIC TRACK CIRCUITS 2 Sheets-Sheet 1 Filed Sept. 22, 1955 HIS' 'TWOHJVEY March 29, 1960 L. DU'rEn. 2,930,887

ELECTRONIC TRACK CIRCUITS Filed Sept. 22, 1955 2 Sheets-Sheet 2 INVENTQR. aan? aec BY www.

" nited States ELECTRONIC TRACK CIRCUITS Louis Duteil, Livry Gargan, France, assignor to Compagnie des Freins et Signaux Westinghouse, Paris, France This invention relates to traffic control, and particularly to an improved track circuit for use in railway signaling systems. i

ln conventional railway signaling practice, widespread use is made of track circuits comprising a relay energized through the rails of a stretch of track and shunted by the passage of a train. In previous circuits of'this type, however, where the track was poorly insulated or where light trains were employed, the value of the ballast resistance might approach that of the shunt resistance, resulting in the failure of the track circuit to function as intended.

Accordingly, it is an object of my invention to provide a track circuit which is positive in operation even though the ballast resistance may be very low compared with the resistance of the shunt afforded by the passage of a train.

It is a further object of my invention to provide an velectronic track circuit operating on the closed circuit principle which is fail-safe in the event of broken rails, severed connections, wet ballast or other adverse contingencies to which signaling equipment is potentially subject.

It is a further object of my invention to provide a coded electronic track circuit particularly adapted to be employed in systems where light trains offering high shunt resistance may be encountered.

It is a further object of my invention to provide an electronic track circuit having high sensitivity to the presence of traflic, but being relatively insensitive to variations in the shunt resistance offered by such traic.

It is a further object of my invention to vprovide an oscillator having a novel control circuit embodying noncritical components. Y i

Other objects and further advantages of my invention will become apparent as the description proceeds.

I will iirst describe an embodiment of my invention, and will then point out the novel features thereof in claims.

In the drawings,

Fig. 1 is a wiring diagram of an embodiment of my invention;

Fig. 2 isKV a diagrammatic sketch at a typical vcross section of a stretch of track, showing the wheels of a train adjacent thereto, and illustrating the equivalent resistance of the various elements; and

Fig. 3 is an equivalent circuit diagram of the structure lof Fig. 1 in which track connections have been replaced by their equivalent resistances.

Corresponding elements are given the same numbers in the various figures.

Referring to Fig. 1, a stretch of track is shown comprising rails 1a and 1b terminated at each end of the stretch by insulated rail joints. An oscillator circuit comprising anelectron discharge device such as tube 11 is connected across rails 1a and 1b at terminals 2 and 3. The output of the oscillator on lead 19 is employed to control relay 25, which when picked up shunts the stretch across terminals 33 and 34 by means of its front Contact tent 32. Relay 25 is picked up and released periodically inV a manner to be hereinafter described, and thereby actuates the decoding circuit comprising relays 40 and 41 which control green and red signal lamps G and R in accordance with traflic conditions. Before describing the circuit of Fig. l in further detail, the system will be described with reference to Figs. 2 and 3, wherein certain equivalent circuit constants are shown which are not readily apparent from the circuit of Fig. l.

Fig. 2 shows a typical cross section of the stretch of track including rails la. and 1b shown in Fig. 1. These rails rest on ties 52 and ballast 53 which in turn is supported by the earth indicated schematically at 56. Portions of wheels 49 and Sii of a train are shown adjacent the rails, but slightly separated therefrom to simplify the description of their electrical properties. The equivalent circuit constants of these elements are shown in dotted lines.

Resistance RS represents the shunt resistance offered by a train having wheels 49 and 50. This value may vary widely in practice depending on the nature of the equipment and its load.V RS may be considered to include both the wheel to rail Contact resistances and the wheel to wheel internal resistance of the train.

Resistors R1 and R2 may be considered to represent the rail to rail resistance including Contact resistances and ballast resistance. Since the rails are symmetrical, the rail to earth resistances will obviously be equal. While these resistances could be represented in the forni of a delta network, for later convenience they are shown as a resistor R3 in a T network, which is well known as the equivalent of the delta network. The ground shown for resistor R3 is the true earth ground in the vicinity of the track. Throughout this description and in all of the figures, it is this potential which is indicated by this ground symbol.

Fig. 3 shows a circuit corresponding to that of Fig. l, but including the equivalent circuit constants of Fig. 2 rather than the track connections of Fig. l. The additional equivalent resistance RT corresponds to the resistance of rails 1a and 1b between the ends of the track stretch.

Referring to Fig. 3, tube 11 is connected as an oscillator having a resonant circuit comprising primary 15 of an output transformer 5S and condenser 14, tuned to the frequency at which it is desired to operate. This frequency is essentially determined by the length desired for the track sections, and will ordinarily be low, of the order of to 1000 cycles. Plate voltage is supplied by a suitable D.C. source 17. Source 17 is shown returned to a chassis ground which is electrically isolated from the earth ground previously referred to. The operating level of the tube is set by cathode resistor 12 and grid resistor 10, which are also returned to the chassis ground.

Output coil 16 of transformer 55 Vis connected through limiting resistor 6 to resistor R1l of the rail network, and at the other end to earth ground. Resistor R3 is grounded as shown.

It will be noted that there are essentially three electrical paths across output coil 16. The rst comprises a load circuit including lead 19, rectiticr 20, resistor 21 and condenser 23 in shunt with relay 25 and resistor 22. The second includes resistor 6, and equivalent resistances R1 and R3 in series. The third includes resistor 6 in series with potentiometer resistor 8. The second and third paths may be considered as a pair of potential dividers in parallel. i

Feedback to the oscillator is provided over a path between intermedite points on the potential dividers including wiper 9, lead 18, resistor 10, chassis ground, resistor 12, condenser 13 and equivalent resistance RZ.

acarrear Current will flow in one direction or the other along this path depending on whether terminal 2 is at a higher or lower potential than wiper 9; that is, the direction of current llow depends on the relative impedance ratios of the aforesaid potential dividers. In the rst case, the cathode of tube ll will be driven to a higher potential relative to the grid and the feedback will be degenerative. In the second case, the grid will be driven to a higher potential relative to the cathode, and the feedback will be regenerative, causing oscillation.

As previously noted, output lead 19 is connected through rectier 20 and resistance 2l, across capacitance 23, through resistance 22 to D C. relay 25 and thence to ground. Capacitance 23 operates to hold up the relay during the negative output half cycle. Resistance 22 is shunted by arm 30 and front contact 29 of relay ,to equalize the pickup and release times of the relay. Arm 31 of relay 25 operates front Contact 32 to shunt Vtrack resistances R1 and R2 with rail resistance RT. Terminals numbered la and 1b to correspond with the Arails are shown connected across contacts 31 and 32 of relay 25, and adjacent thereto is shown shunt resistvance RS having terminals numbered 49 and 50 to correspond with the wheels shown in Fig. 2. Terminals 49 and 50 may be considered to engage terminals la and 1b at times when a train is on the stretch.

As set forth above, whether or not tube il will oscillate .depends on the relative impedance ratios of the potential dividers, which determine the potential of terminal 2 relative to wiper 9. The values of capacitor i3 .and resistance 8 are so chosen that some adjusted pos"- tion of wiper 9 on resistance S can be selected which will make the circuit oscillate with relay in the position ,shown and with open terminals at la and lb. Under these conditions, terminal 2 will be lower in potential .than wiper 9, and the ratio of resistance R1 to R3 vwill .he less than the ratio of the portion of resistance 8 between wiper 9 and terminal 3 to the portion between wiper 9 and ground. When relay 25 operates to shunt R1 and R2 with RT, however, or when a train enters :the section and R1 and R2 are shunted with RS and RT, -the equivalent resistance between terminal 2 a-nd terminal 3 will be lowered, modifying the impedance ratio of the .divider comprising resistances R1 and R3, and the potential of terminal 2 will rise relative to wiper 9, reversing the phase of current in the kfeedback path and causing oscillation to cease.

When. oscillation begins, relay 25 will be energized to close its front contact 32 and apply resistance RT between terminals 2 and 3. The result of shunting resistances R1 and R2 with resistor RT is to stop oscilla -tion as described above. When this happens, relay 25 drops away and the circuit again begins to oscillate. it will be seen that this action produces a coding operation of relay 25.

Should resistance RS be placed across terminals 1a and lb, by the passage of a train as described above, oscillation would be prevented regardless of the dropping out of relay 25'until shunt RS was removed.

Resistance RT is ordinarily relatively small compared with ballast resistances Rl and R2. It has been included in the equivalent circuit merely to illustrate the vcontinuity checking feature incorporated :in the system, `whereby a broken rail would operate to prevent any shunt of the rails, and would maintain relay 25 continuously energized to provide a safety warning opera tion.

Shunt resistance .RS may vary in value from a very .low level associated with large heavy trains, to avery high level associated with light train and ,rusty rails. However, since the shunt resistances RT vand RS are compounded with ballast resistances Rl and R2 in parallel, it can be seen that their elfect on the resistance between .terminals .3 and 2 will be relatively great, and that even .though these shunt values might vary widely, vit will still be possible to design the oscillator such that oscillation will always occur without the shunt but will never occur when the shunt is present. Therefore, the circuit is highly sensitive to the presence of vehicles .having widely varying shunt resistances, but is relatively insensitive to varying operating conditions such as the state of the ballast, the resistance of the rails and the train shunt resistance.

Referring again to Fig. l, terminals 2 and .3 are connected across the rails la and lb at one end of the track stretch, through lead 4 to condenser 13 and through lead 5 to resistor 6, respectively, to constitute an input circuit corresponding to that shown connected to terminals 2 and 3 in Fig. 3. Terminals 33 and 34 at the opposite end of the track stretch are connected to contacts 31 and 32 of relay 2S to shunt the track section, under conditions of rail continuity, in the manner described in connection with Fig. 3.

In addition to front contact 29, for the purpose of adjusting the relay drop-out time, relay 25 has an additional arm 35 connected to a source of voltage indicated schematically at B which is employed to energize relay 41) through lead 38 and front contact 37, or relay 41 through lead 39 and back contact 36. Relays 49 and 41 are of the fast pick-up, slow release type which is well known in the art, the characteristics of which are indicated by the heavy base lines. These relays have a com mon terminal 42 connected to the opposite side of source of voltage B as indicated by C.

Relays Llil and Si operate as a conventional decoder incorporating safety features, such as is well known in vthe art. Relay al is normally held up through back contact 36 of relay 2, connecting battery B through front contact 44, over back contact 47 of relay 4d through red signal lamp R and hack to the source return C. `Upon energization of relay 25, front contact 37 of relay 25 will be picked up and relay 40 will pick up' to engage front contact 43. Since relay 41 is of the slow release type, there will be time for the energization of green lamp G before relay 4l drops away.

Under normal conditions, in the absence of trahie,

relay 25 will operate at a rate sufficient to maintain both relays 4l) and 4l. picked up, so that the green light will be illuminated. However, when a train enters the track section and shunts rails la and 1b, the oscillator will cease operation regardless of the dropping out of relay 25 and relay 25 will remain deenergized until the train leaves the section. Accordingly, relays 40 and 41 will eventually drop and remain deenergized, energizing red lamp R through back contact 4S and back contact 47.

The operation of my invention may be summarized briefly as follows:

Under normal conditions, Wiper 9 is set such that the ballast resistance between terminals 2 and 3 is vsuiicient to permit oscillation in the circuit of tube 11. When the tube goes into oscillation, relay 25 is picked up, shunting rails 1a and 1b at terminals 33 and 34 over front contact 32. This raises the potential at terminal 2 with respect to wiper 9 and the feedback is reversed so that oscillation ceases, and the relay drops away.

With the relay deenergized, oscillation recommences, and this operation continues in a cyclic manner at a frequency determined by the characteristics of relay 25 and control networks 2l, 22 and 23 to produce a code following operation'of relay 25. However, a train shunting rails la and 1b will stop oscillation in a manner independent of the operation of relay '25, and continue to hold relay 25 deenergized while the train is in the stretch, regardless of the resistance offered by the train imposed shunt.

While i have shown relay 2S as operating a front contact to yshunt the track, it will be apparent that by interchanging the connections from feedback ycondenser' '13 and lfeedback resistance d to the rails, a back contact :of vrelay 425 vcould be used to operate the-shunt, the elect 523 S3 of which would then beto increase regenerative feedback and begin oscillation. In such a case, the presence of a train shunting the rails would cause oscillation to continue throughout the presence of the train in the stretch, and cessation of oscillation would give an indication of loss of occupancy of the stretch.

iIt should be noted that While l have illustrated electron discharge device 11 as a conventional vacuum tube, any equivalent device having electron emitting and receiving electrodes and a control electrode might be used in place of a tube. For example, a transistor might be employed for this purpose Without departing from the scope of the invention.

While I have described an embodiment of my invention in detail, it will be apparent that many modiiications and `changes therefrom could be made within the scope of the invention. Accordingly, I do not wish to be limited to the details shown, but only by the scope of the following claims.

Having thus described my invention, what I claim is:

l. An electronic track circuit for a section of railway track having ballast resistance existing across the rails, comprising in combination; an oscillator for generating signal energy, said oscillator having input and output portions; a first feedback path for said oscillator comprising a first means connecting signal energy from said output portion to one of said rails, the ballast resistance and a second means for connecting signal energy from the other rail to said input portion; a second feedback path for said oscillator comprising adjustable impedance means connecting signal energy from said output portion to said input portion; said impedance means being adjusted such that the combined signal energy connected through said feedback paths maintains said oscillator in oscillatioma relay including an operating coil and a plurality of contacts, each contact having at least a heel and front contact point; means for connecting said coil to receive signal energy from said output portion; means for connecting a heel of one of said contacts to one rail and the front contact point of said one contact to the other rail; said relay coil upon receiving energy from said output portion actuating said contacts to close and' thus shunting said ballast resistance of said first feedback path for stopping oscillations in said oscillator regardless of the adjusted position of said impedance means; and indicating means controlled by a second of said relay contacts for providing an indication of the operation of the track circuit.

2. Relay coding apparatus, comprising, in combination, an oscillator generating signal energy at a given frequency, said oscillator having an electron emitting electrode, `a control electrode and an electron receiving electrode, a resonant circuit tuned to said given frequency and connected to receive energy from said electron receiving electrode, an output circuit inductively coupled to receive energy from said tuned circuit, rst and second potential dividers connected in parallel with one another and connected to receive energy from said output circuit, said first divider comprising a iirst impedance connected in series with a second impedance, an adjustable wiper on said second divider, means for connecting said electron emitting electrode to the junction of said first and second impedances, means for connecting said control electrode to said wiper, said control electrode receiving energy from said electron receiving electrode through said tuned circuit, said output circuit and said rst divider and said wiper to maintain said oscillator oscillating, a relay, means for connecting signal energy from said output circuit to said relay when said oscillator is oscillating, and means actuated by said relay when said relay is energized for shunting said first impedance of said first divider whereby the energy received by said control electrode is changed and said oscillator stops oscillating.

3. An electronic track circuit for a stretch of railway track having ballast resistance existing'across the track rails, comprising in combination, a :relay including an operating coil, an armature actuated by said coil, and movable contacts controlled by said armature, said relay contacts having a first position in which said contacts are connected across the rails at one end of said stretch and a second position in which said contacts are opened; an oscillator of signal energy having input and output portions; iirst means for connecting signal energy from said output portion to one of said rails at a point at the other end of said stretch; second means for connecting signal energy from the second of said rails at a point at said other end of said stretch to said input portion; said first connecting means, the ballast resistance and said second connecting means providing a first feedback path for said oscillator; an adjustable impedance means connecting signal energy from said output Vportion to said input portion for providing a second feedback path for said oscillator; the combined signal energy connected through said two feedback paths maintaining said oscillator in oscillation; means for connecting signal energy from said output portion to said coil of said relay; and said relay contacts being movable to their first position by said armature when said coil receives signal energy for shunting the ballast resistance thereby changing the signal energy connected from said output portion through said feedback paths to said input portion to cause said oscillator to stop oscillating and when said oscillator stops oscillating said relay coil being deenergizcd causing said armature to move said contacts to a second position to remove the shunt thereby causing said oscillator to again start oscillating for obtaining a coding action.

4. An electronic track circuit for a stretch of railway track, comprising in combination, an oscillator for providing signal energy at a given frequency and having an electron emitting electrode, a control electrode and an electron receiving electrode; a resonant circuit tuned to said given frequency and connected to receive signal energy from said electron receiving electrode; an output circuit inductively coupled to receive signal energy from said tuned circuit; first means for connecting signal energy from said output circuit to a first rail of said stretch of track, second means for connecting signal energy from said second rail to said electron emitting electrode; said lfirst connecting means, the ballast resistance and said second connecting means providing a feedback path for said oscillator; a relay including a plurality of pairs of contacts and a coil for operating said contacts, said operating coil being connected to receive signal energy from said output circuit; and means for connecting a contact of one pair of contacts to one rail and the other contact of said pair to the other rail; said contacts being actuated to close when said relay coil is energized to shunt a part of said feedback path whereby said oscillator stops oscillating.

5. An electronic track circuit for a stretch of railway track, comprising in combination, an oscillator for generating signal energy at a given frequency and having a control electrode, an electron emitting electrode and an electron receiving electrode; a resonant circuit tuned to said given frequency connected to receive signal energy from said electron receiving electrode; an output circuit inductively coupled to receive signal energy from said tuned circuit; first means connecting signal energy from said output circuit to a iirst rail to said stretch of track; second means connecting signal energy from the other rail of said stretch of track to said electron emitting electrode; said first connecting means, the ballast resistance and said second connecting means providing a first feedback path for connecting signal energy from said electron receiving electrode to said electron emitting electrode; impedance means for connecting signal energy from said output circuit to said control electrode providing a second feedback path for coupling signal energy from said electron receiving electrode to said control electrode; the combined signal connected through said .two feedback paths maintaining said oscillator in oscillaassess? tion; a relay having a plurality of pairs of contacts and a coil for operating said contacts, andsrneans for connecting signal energy troni said output circuit to said coil, a first pair of contacts being connected in parallel to said ballast resistance, said lirst pair of contacts being actuated to close when said relay coil is energized for shunting said ballast resistance whereby said discharge device ceases oscillation.

6. A control network, comprising in combination, an oscillator tube Afor generating signal energy at a given frequency, said oscillator including electron emitting electrode, a control electrode and an electron receiving electrode; a resonant circuit tuned to resonance at said given frequency connected to receive signal energy from said electron receiving electrode; an output circuit inductively coupled to receive signal energy from said resonant circuit; rst and second potential dividers connected to receive signal energy from said utput circuit; iirst and second means connectingsaid first and second dividers to said control electrode and said electron emitting electrode respectively to provide a feedback voltage for said oscillator; the effective feedback voltage being related to' the ratio of the voltage coupled by tirst connecting means to said control electrode and the voltage coupled by said second connecting means to said electron emitting electrode; and means for shunting a part of one of said dividers to modify said voltage ratio -to cause said discharge device to cease oscillating.

7. A control network comprising in combination an oscillator for generating signal energy at a given frequency, said oscillator having input and output portions, 'first and second potential dividers connected in parallel to one another and connected to 'receive signal energy from said output portion, rst and second means connecting points intermediate the ends of said lirst and second dividers respectively to said input portion for-providing a pair of feedback paths for connect'ng signal energy from said output portion to said input portion to sustain oscillations in said oscillator and means for shunting a portion o' said second divider for modifying the feedback energy coupled to said oscillator to cause said oscillations to cease.

8. An electronic track circuit for a stretch of track including Va pair of rails having the usual ballast resistance existing thereacross, said track circuit comprising a generator of oscillating signal energy, said generator` having input and output circuits, feedback means for said genrat r comprising a pair or electrical paths for coupling energy from said output circuit to said input circuit; the rst of said paths comprising a first means for connecting said output circuit to one of said rails and the ballast resistance and a second means for connecting said other rail to said input circuit; the second of said paths comprising impedance means for connecting said output circuit to said input circuit; and means for controlling the relative amount of energy co'upled from said output circuit to said input circuit through each of said two feedback paths for controlling oscillations in said generator.

9. Apparatus for use in railway signaling systems for a stretch ol track including a pair of rails having the usual ballast resistance existing thereacross comprising a generator o'f oscillating energy, said generator having input and output circuits, feedback means comprising a pair of electrical paths for coupling signal energy from said output circuit to said input circuit `for sustaining oscillations in said generator, the rst of said paths comprising a first means connecting said output circuit to one or" said rails and the ballast resistance and second means connecting said other rail to said input circuit, the second of said paths comprising impedance means for connecting said output circuit to said input circuit, a relay having an operating coil, means for connecting said relay co'il to said first connecting means for receiving energy therethrough, means for shunting said ballast resistance, said shunting means being actuated when said relay coil is energized thereby producing a change in the relative amount of energy coupled through each of said two yfeedback paths to cause said generator to' stop oscillating.

References Cited in the tile of this patent UNITED STATES PATENTS 2,602,139 Hod er et al. July 1, 1952 2,715,718 Holtje Aug. 16, 1955 2,768,336 Shoultes et al. Oct. 23, 1956 2,773,220 Aron Q Dec. 4, 1956 

